Discussion of the Background
Chemical reactors employing beds of solid catalyst particles are used to conduct many useful industrial processes. Such packed bed rectors often have a large thermal inertia, and can take many hours to reach their operating temperature during process startup. This is especially true if the packed bed reactor is used to promote a gas phase reaction where the thermal mass flux of the reactant gases is low relative to the thermal inertia of the packed bed.
An especially deleterious condition can result when the packed bed reactor is used to process a condensable vapors such as water or hydrocarbon vapors. In this case, if the bed has not reached the boiling temperature of the condensable species, liquid formation is the inevitable result. A prime example of such a process is the water gas shift reaction, where water vapor is reacted with carbon monoxide to produce hydrogen and carbon dioxide. In reactions such as water gas shift, the condensed liquid can be subsequently vaporized on the catalyst particles, which are generally porous, and have relatively low mechanical strength. This vaporization can generate relatively extreme mechanical stresses inside the catalyst particles, and can lead to their mechanical failure. The fractured catalyst particles can subsequently lead to severe operational difficulties such as fouling or plugging of the bed or of downstream process elements.
It is possible to forestall this condensation and subsequent vaporization and catalyst failure by heating up the packed bed using a stream of non-condensable vapor. This requires a ready supply of such vapor on hand. Since many catalysts are sensitive to exposure to oxygen, this generally means supply of an inert fluid. This undesirably increases the complexity of the process plant. Again, water gas shift reactors are a prime example as they typically employ air-sensitive catalysts.
Alternative methods employed to heat packed bed reactors during startup have included heating the reactor with second fluids such as heated oil or steam through a heat exchange loop. These methods are advantageous if such heating fluids are readily available, but increase system complexity undesirably if they must be provided solely for heating up the packed bed reactors. Alternatively, commercially-available electrical heating elements may be provided. Examples of such elements include band, or barrel heaters which may be attached to the outside of the reactor. These elements must transfer heat through the reactor vessel wall, undesirably requiring that wall to have good heat transfer properties. Further, even with good insulation, much heat applied in this fashion is lost to the environment, increasing the amount of electrical energy required to heat up the reactor.
Immersion heating elements are also readily available, and these may be submerged directly into the catalyst bed. Placing the heating elements within the catalyst bed offers obvious advantages in the amount of heat supplied directly to heating the catalyst bed relative to the fraction lost to ambient. Immersion heating elements present other special problems though, as their high rate of heat input can impart significant thermal stresses on individual catalyst particles. These particles, which are generally constructed from brittle ceramic materials, are susceptible to fracture under high thermal stress. Thus, application of immersion heaters is limited to heaters of low heat output to minimize thermal stresses. This undesirably increases the number of heating elements required to obtain an acceptable rate of heating without causing catalyst failure.